The background to the invention will now be described with reference to the accompanying Figure in which:
FIG. 1 is a block diagram of a typical wireless telecommunication system.
Referring to FIG. 1, a typical wireless telecommunications system divides a geographical area into a plurality of cells 10 each of which comprises one or more base stations (Node B), wherein mobile user devices (UE) located within a cell 10 communicate with the base station (Node B) of that cell 10. More specifically, a base station (Node B) handles radio transmissions and reception to and from mobile user devices (UE) over a radio channel (R1), and is controlled by a radio network controller (RNC) that is connected to a core network (20).
In addition to radio channels (R1) for the transmission of voice or data traffic, a wireless telecommunications system also provides one or more channels for the broadcast of synchronization messages by mobile user devices. These broadcast messages enable the mobile user devices to synchronize their transceivers to the frame/slot/bit structures of a given base station.
Code Division Multiple Access (CDMA) is a multiple access technology in which users are separated by unique codes, so that all the users can use the same frequency and transmit at the same time. Recent developments in signal processing have made it possible to use CDMA for wireless communication. Wideband Code Division Multiple Access (WCDMA) is a recent advancement in CDMA technology that uses a wideband carrier signals.
Returning to the generic model of the wireless telecommunication system depicted in FIG. 1, WCDMA specifies the physical random access channel (PRACH) for the transmission of synchronization messages by a mobile user device (UE) to a base station (Node B) (wherein a random access channel (RACH) is the transport channel mapped to the PRACH).
The synchronization between a mobile user device and a base station is based on a hand-shaking type process, in which the mobile user device transmits a synchronization message containing an identification code (known as a preamble) to the base station. On receipt of the message, the base station compares the message's identification code with a set of predefined identification codes. If the base station recognizes the identification code of the synchronization message, it transmits a confirmation message back to the mobile user device. The receipt of the confirmation message by the mobile user device enables the device to estimate the propagation delay in the communication link between it and the base station.
In practice, a base station recognizes a preamble in a received signal, by correlating samples from the received signal with a known preamble scrambling code and preamble signature sequence. A RACH preamble is typically 1 msec in duration. This imposes a severe time constraint on the process of recognizing the preamble and is particularly problematic if the mobile user devices randomly use a number of different preambles, because a base station must correlate a received composite signal with all possible preamble scrambling codes and preamble signature sequences.
Delay spreading is a type of distortion caused when multiple reflections of a signal arrive (via different paths) at a base station at different times. The distortion results in the spreading or “smearing” of the received signal. Since the maximum delay spread of a preamble may differ depending on the cell size of a base station, the computational load of recognizing a preamble will also vary depending on this feature.
At present, the recognition of preambles is performed in the time domain with special hardware accelerators. However, it is known that correlation can also be calculated in the frequency domain. US Patent Application US2004042388 describes a correlation detection apparatus that receives a RACH signal from a mobile user device and subjects the signal to a Fast Fourier Transform (FFT). The apparatus also subjects RACH preamble codes stored therein to an FFT and multiplies the resulting spectrum of the RACH preamble codes with the spectrum of the received signal. The apparatus then transforms the result of the multiplication operation into the time domain (by means of an Inverse Fast Fourier Transform (IFFT)) to obtain a delay profile.
However, the apparatus described in US2004042388 requires a large amount of memory to process a signal. Furthermore, the apparatus does not improve the latency aspects of the preamble recognition process. Similarly, the apparatus described in US2004042388 uses a special FFT algorithm rather than a conventional FFT and/or IFFT algorithm. Finally, the apparatus described in US2004042388 does not merge the FFT and Fast Hadamard transform (FHT) for RACH signals.